1. Field of the Invention
This invention relates to methods and systems for determining the attitude of an object. In particular, but not exclusively, the invention relates to such systems and methods for determining, without ambiguity, the attitude of an object by means of a radio or other electromagnetic wave signal or signals transmitted from a single source position. The object may be, for example, a vehicle such as an aircraft, or spacecraft, or a projectile such as a guided missile or a course-corrected shell.
2. Discussion of Prior Art
It is often desired to determine the attitude of a vehicle with respect to some fixed axis system. This may be required to direct sensing devices, or radio or optical beams in desired directions. Alternatively control means required to direct the vehicle on a desired course may require that the vehicle be set to a prescribed attitude. Methods exist, e.g. Inertial Navigation, by which vehicle attitude can be obtained without the use of radio systems. It may however not be convenient to employ such methods
FIG. 1 of the accompanying drawings shows a conventional radio attitude determination system which allows attitude determination in only two of the three axes. Radio signals from a transmitting antenna T in a known location are received by an antenna V on the object whose attitude is to be determined. By conventional tracking methods, the resolved components (.alpha.,.beta.) of the angle between the direction of arrival of the radio signals and an axis (X) which is fixed in the object, e.g. the boresight of the receiving antenna V, are determined. As noted, in this basic form the attitude of the object is not completely determined, since rotation about the X-axis is not measured.
The attitude in a third axis may be determined in various known ways but each has disadvantages. For example, two or more transmitting antennas may be provided in known different sites. The angular components between the directions of arrival of the radio signals and axes which are fixed in the object are determined. The angle measurements are then employed to calculate the complete attitude of the object. In this method at least two different sites are needed for the transmitting antennas and this may be a major problem; for example at sea only a single ship may be available to carry the transmitting antenna.
In another method for increasing the completeness of attitude determination, a single antenna transmits a beam which is elliptically polarised with a known major axis direction. The receiving antenna is designed to measure the orientation of the major axis of the received polarisation relative to the antenna. This method allows the attitude to be determined in three axes but there is a 180.degree. ambiguity in the angle which is measured by polarisation and so an additional method is needed to resolve the ambiguity.
A need exists therefore for a method and system which permit unambiguous determination of the attitude of an object by a radiation beam, using only one location for the radiation source or sources.